The art of etching metal plates in order to produce a reproducible image is centuries old. The basic principle involves putting a resist coating on the surface of a clean smooth metal plate, removing a portion of this resist with a suitable tool such as a needle and then immersing the metal plate for a predetermined time in an acid bath in order to bite or remove a portion of the metal which is exposed thereby. The resist is then dissolved off, usually by means of a solvent, and a printing ink rubbed into the surface of the plate. The plate is then rubbed with a cloth to remove all or substantially all of the ink that does not reside within the grooves caused by the etching process. The plate is then laid face up on a suitable surface, covered with a suitably prepared, usually moist paper sheet and pressure applied thereto, usually by means of roller press. This procedure causes the ink to be transferred from the grooves in the metal plate on to the paper to give the printed image.
These techniques have been used to create deep and wide cuts in the plate to provide an effect on the paper known as embossing.
In a well known variation of the acid etching process, known as aquatinting, the resist does not totally and completely cover the metal plate. There are various methods for producing aquatint. The most common of these is to deposit a thin dust film of rosin on the plate and heating the plate just enough to make a major portion of the rosin adhere to the plate but not enough to produce a uniform coating. When this plate is placed in an acid bath the acid will attach those portions of the metal to which the rosin does not adhere. Other methods of aquatinting are well known to those skilled in the art of graphic printing. The metals generally speaking, used to produce etchings are zinc or copper, brass and steel have also been used, bronze and iron can also be employed but are not as favored.
A further embodiment of aquatinting is known as sugar lift wherein a mixture of syrup, tempera paint and soap flakes is painted onto a rosined plate, the painted plate placed first in water, to achieve the lift, and then in acid to provide a very "soft" printable image.
Whatever metal is used the general principle is the same. In order to achieve the etching or removal of metal rather strong acid media are employed. These can be either nitric acid or a medium generally known as "Dutch mordant" which comprises hydrochloric acid and potassium chlorate as its main constituents. Both etching solvents require substantial ventilation to protect the worker from the fumes which are generated in the process. Unfortunately, it has been found that artists who practice these processes are not sensitive to the health dangers involved and work directly above the acid baths in order to carry out certain brushing steps to obtain the bite which they desire. The provision of acid proof masks is not generally practical and if available would usually not be employed by artistic workers. Furthermore, the exhausted baths, that is to say baths whose content are still acidic but are not longer of sufficient strength to be useful in the etching process must be disposed of by steps of neutralization which are expensive and often ignored. Furthermore, even if neutralized the baths still contain large quantities of metal which, where copper is a content of the metal, are exceedingly environmentally harmful.
The rather dangerous nature of the etching process has therefore, restricted its use to the professional level and in institutions of higher learning. The principle of etching however, would be exceedingly instructive to younger students if a methodology could be made available which was totally safe for unskilled persons such as students of grammar school or high school age.
It is well known that where a metallic plate is placed in an electrolytic bath having another electrode and a source of direct current is applied to said electrodes through said electrolytic bath in such a way that a metallic plate becomes the anode, metal irons will pass from the anode to the other electrode (cathode). It was recognized at a very early stage that this principle could be utilized to create etched plates, for example, Schwuchow and Johnston, U.S. Pat. No. 1,047,995, who utilized zinc half-tone plates at a current of about 10 volts for from about 1 to 2 minutes. It was recognized by Holland in U.S. Pat. No. 2,074,221, that the efficiency of anodic etching could be increased by agitating the plates and a further mode of agitation was provided by T. F. Johnstone, in U.S. Pat. No. 2,110,487, in which a blast of air was bubbled through the electrolytic medium as an agitating means.
Corbet, in U.S. Pat. No. 2,536,912, recognized that under the rather vigorous conditions which he utilized, namely, etching at 6 volts utilizing a current of approximately 35 amperes, the pH of the solution tended towards the basic side and that is was desirable to maintain the slightly acidic nature of the electrolyte by the addition of acid. Other workers such as Raviv, et al., U.S. Pat. No. 3,635,805 and King, et al., U.S. Pat. Nos. 3,843,501 and Inverso, 4,098,659, have utilized the principle of metallic etching for very deep cutting of metal, analogous to utilizing a lathe without the currents of metallic structure deterioration due to the heat generated in such lathing processes.
Nee et al. U.S. Pat. No. 4,729,946 discloses a method of etching discs to be used as laser-read compact discs which had previously been plated with a thin layer of copper. Parts of the copper plate are were covered with a photo resist. It is specifically stated in the specification that this copper layer is fine grained. Thus this copper layer does not have the courser grained structure of metals items which are derived from the molten state such as cast objects or plates rolled from ingots. The exposed portions were electrolytically etched out to a predetermined depth by connection to the anode of a direct current source of about 6 volts. The electrolyte used was an alkaline medium containing alkali metal or ammonium cations. It is further noted that this procedure requires a cathode bag to catch the copper "plated" to but not retained by the cathode. Such non-adhesion is characteristic of electrolytic cells operating at such relatively high voltages.
Notwithstanding the aforementioned patents directed to anodic etching, there is no mention of anodic etching as a suitable graphic arts process in any old or recent text directed to printing methods for artists. In particular, the recent well accepted major treatises entitled Printmaking, History and Process by Saff & Sacilotto, Holt Rinehart & Winston, New York, 1978 ISBN 0-03-042106-3 and Complete Printmaking, Ross et al., (rev. ed) Free Press, New York, 1989 ISBN 0-02-9273714, make no mention of anodic etching.
The problem with the anodic etching processes of the prior art is that they operate at high voltages and rather substantial current levels, which give rise to the generation of gases such as oxygen and hydrogen, which in certain concentrations, when mixed, are exceedingly explosive and therefore would create a hazard in the work place where electrical sparks cannot be avoided.
In the electroplating arts, voltages are kept under about 2 v., since the generation of hydrogen bubbles at the cathode where the plating is deposited, interferes with a smooth, well-adhering deposit. It would therefore be desirable to create a process and design an apparatus wherein it was possible to reproduce the effect on a metal plate of traditional etching techniques, which would include not only reproduction of exceedingly fine lines such as those obtained by the non-acid etching procedure generally known as dry-point, to the variously deep engraved lines obtained in traditional etching processes, (i.e., intaglio) to the more vigorous removal of metal in such processes known as the production of embossing plates, wherein depths exceeding 1 mm. are achieved in the plate. Such a methodology should also include the availability of surface modifications techniques which are traditionally known as aquatinting and sugar lift.